This invention relates to a system of controlling radio channels of a mobile radio telephone system.
With increased activity of human living, the requirements for communicating between vehicles such as motor cars, ships, trains, etc. by utilizing existing ordinary telephone networks, have been increasing and various mobile radio telephone systems have been developed to fulfil such requirements.
Among these systems, a system is now being used wherein a service area of the system is divided into a plurality of radio zones, a mobile-system base station (MBS) is installed in each radio zone, and the mobile-system base stations are connected to a mobile-system control unit (MCU) through a common control channel and speech channels each consisting of four lines whereby the mobile-system base stations are connected to an ordinary telephone network through the mobile-system control unit. Such a system is described in U.S. Pat. No. 3,663,762 dated May 16, 1972 and Z. C. Fluhr and E. Nussbaum paper of the title "Switching Plan for a Collular Mobile Telephone System" I.E.E.E. Transaction on Communications, Vol. COM-21, Nov. 1973. The reason for using a common control channel in this system is to improve the control efficiency and the efficiency of utilization of the speech channels by commonly controlling a plurality of speech channels at a high speed where a mobile communication service is contemplated in a high capacity and over a wide area.
However, the prior art system is accompanied by a number of problems so that its reliability is not yet sufficiently high as will be described hereunder.
More particularly, the mobile-system base stations, the mobile-system control unit and the mobile-system exchange unit are interconnected by a four line speech channels and a control channel common to respective speech channels, and the information for controlling the processing of origination and paging are transmitted through a common control line or channel. Moreover, the information transmitted through the common control line is in the form of digital information. Although digital information is efficient for high speed and high density transmissions it is liable to be affected by noise. For this reason, where digital information is used, it is usual to use an expensive and complicated check circuit in order to assure extremely high accuracies. The use of such check circuit, however results in the complication of the system control and in the increase in the cost of installation. Furthermore, where the information regarding the state of the radio channel of each speech channel is to be transmitted continuously, sending this information over a common control channel may be difficult due to the large volume of information which must be sent to represent the constantly changing state of the radio channel.
In addition, since the radio telephone system utilizes wireless waves as the information transmission medium, it involves various problems specific to radio transmission. Firstly, due to decrease in the transmission characteristics caused by fading phenomena control information sent to or received from mobile vehicles will not be transmitted accurately, or when a speech channel is available between a mobile vehicle and a mobile-system base station it will be difficult to maintain desired speech quality. In addition, under these conditions, an electric wave utilized in another radio zone may enter into an idle speech channel or may cause interference with waves of the adjacent zones.
The state of the radio channel for each speech channel is represented by the presence of an electric wave which indicates whether the channel is available or not, and the mobile-system base stations, mobile-system control unit and mobile-system exchange unit are required to acknowledge such information. In view of the large volume of information required to represent the constantly changing state of the speech channels, however, it is impossible to transmit such information over the common control channel. However, the provision of control lines for transmitting digital signals corresponding to respective speech channels increases the size and cost of installation.
Due to these problems, among various operations of the system, selection of an idle speech channel is most severely affected. Generally, to select an idle speech channel, an idle speech channel memory device (or a temporary memory device) adapted to store an idle or busy state of the speech channel is provided for finding an idle speech channel out of a plurality of speech channels where a request for origination or paging is made. In the prior art system, it is impossible to change the channel content of the idle speech channel memory device except a case wherein the channel content is changed to a blocked state for the purpose of designating a new channel and a case wherein the channel content is changed to idle state when the speech of a given channel is terminated.
For this reason, where the channel content has been changed by an erroneous control data, even when the controller of the mobile-system control unit designates a speech channel based on the content of the idle speech channel memory device it may happen to designate a busy speech channel or to fail to designate a channel when the channel content is blocked irrespective of the fact that it is actually in an idle state. When an actually idle speech channel is designated under a poor wave transmission condition, it would be impossible to maintain the desired speech quality. Where the wave transmission condition becomes poor during speech and irrespective of the fact that the speech has already been terminated the speech channel might be continuously disabled when the mobile control unit fails to detect termination of the speech. Under these conditions, the mobile vehicle and the mobile base station continue to radiate electric waves.
Another problem is that the reliability of all speech channels of a given mobile base station would be decreased when there is some transmission trouble, for example noise, wire breakage, etc., occurs on the common control channel.
Still another problem is that when it becomes impossible to generate control digital data due to a system trouble of the radio mobile control unit, control of the transmitter of the mobile-system base station becomes completely impossible.